One of the most useful tools for the study of developmental genetics in Drosophila is the Ga14-UAS modular expression system. This system allows the use of a wide variety of Ga14 expression lines to express transgenes in diverse tissues. The purpose of the research proposed here is to modify this system with a second level of negative control to allow the activation of a transgene by Ga14 in a set of Drosophila tissues to be tailored to a specific subset of tissues. Repressors based on the bacterial DNA-binding proteins, TetR and LexA, will be made by fusing their DNA-binding domains with repression domains from the Drosophila proteins Hairy and Groucho. The TetR-based repressor function should be regulated by administration of tetracycline, allowing the modulation of Ga14-mediated activation at different developmental stages. The LexA-based repressor will be regulated transcriptionally, such that only tissues with the Ga14-activator but not the LexA-repressor will be capable of expressing a transgene.